Why is there a custom to introduce young children to the study of חומש with ספר ויקרא rather thanבראשית ספר? The מדרש in ויקרא רבה ז,ג answers, let the pure ones come and study the laws of purification." The עץ יוסף explains, Hashem considers one's studying the laws of sacrifice as though he had actually offered that sacrifice (see מנחות קי.). Therefore, when the laws of sacrifice are studied by young children who are pure and unblemished, Hashem regards their learning as though unblemished sacrifices were brought on the מזבח.
On the first פסוק of פרשת ויקרא Rashi explains that the voice of Hashem was loud but only Moshe heard it. Rav Moshe Feinstein in his דרש משה asks a simple yet brilliant question, if the voice of Hashem could not be heard to all of בני ישראל, for what purpose was it so loud? Rav Moshe quotes a גמרא on דף ב. of מסכת סוטה which says that a Heavenly voice announces before a boy is born the daughter of so and so is designated to so and so. In this case too of what purpose is a voice which cannot be heard?
Rav Moshe answers, we should have been able to hear it if it were not for the fact that we were unworthy. The purpose of the heavenly voice is that we should feel that we are being commanded directly by Hashem. The same is true of the Heavenly voice which announces the daughter of so and so is designated to so and so. This special command teaches us in addition to the מצוה to marry, each man has a special מצוה to marry a specific woman.
This is similar to a גמרא in מסכת נדה לא. which says that while a baby is in the mother's womb he is taught the entire תורה but just before he is born he is hit above the lip and forgets everything he learned. Rabbi Joseph B. Soloveichik asked, what is the use of studying all the תורה to just forget it? He answers that if you learn something even if you forget it will make it easier to re-learn the material at a future date.
May we learn this important lesson from our פרשה and teach it to our young children as they begin their study of חומש as the משנה in מסכת אבות (פרק יכ משניות כב-כו) says בן חמש שנים למקרא, a five year old should start to learn תורה and let the pure ones start here and continue learning for the rest of their lives. The learning of the children at this time tender age is vitally important for all of כלל ישראל. חז"ל teach that "אין העולם מתקיים אלא בזכות הבל פיהם של תינוקות בית רבן" (שבת קיט:) - that the world exists because of the merit of the pure breath of learning of small children.